An ATP-independent DNA strand-transfer protein has been identified, purified and characterized from both mitotic and meiotic Saccharomyces cerevisiae cells. We have named them STPalpha and beta, respectively. Using antiserum and the partial amino acid sequence information from the purified proteins, the respective genes have been cloned and sequenced. Although the STPalpha gene (DST1) is not required for mitotic growth, it is required for normal level of meiosis-specific homologous recombination. On the other hand, the STPbeta gene (DST2) is required for meiosis and the gene disruption mutation of the gene exhibited reduced level of meiotic homologous recombination as well as lower level of mitotic homologous recombination. These data have proven that STP is one of homologous recombination components in yeast and an ATP-independent DNA strand transfer reactions catalyzed by STP are biologically important. Furthermore, we have shown that RAD3 protein, at least in vitro, interacts with STP proteins and stimulates DNA strand-transfer reactions by an ATP-independent manner. In addition, antiserum against yeast RAD6, RAD18, and REV3 gene products have been raised to establish a protocol for their protein purification.